The present invention relates generally to magnetoresistive random access memory (MRAM), and more particularly to a MRAM cell structure with a blocking layer for avoiding short circuits.
MRAM is a type of memory device containing an array of MRAM cells that store data using their resistance states instead of electronic charges. Each MRAM cell includes a magnetic tunnel junction (MTJ) unit whose resistance can be adjusted to represent a logic state “0” or “1.” Conventionally, the MTJ unit is comprised of a fixed magnetic layer, a free magnetic layer, and a dielectric tunnel layer disposed there between. The resistance of the MTJ unit can be adjusted by changing the direction of the magnetic moment of the free magnetic layer. When the magnetic moment of the free magnetic layer is parallel to that of the fixed magnetic layer, the resistance of the MTJ unit is low, whereas when the magnetic moment of the free magnetic layer is anti-parallel to that of the fixed magnetic layer, the resistance of the MTJ unit is high. The MTJ unit is coupled between top and bottom electrodes, and an electric current flowing through it can be detected for determining its resistance, and therefore the logic state of the MRAM cell.
One drawback of the conventional MRAM cell structure is that an undesired short circuit often occurs when forming a contact on the top electrode. For example, FIG. 1 illustrates a cross-sectional view of a conventional MRAM cell structure 100 where an undesired short circuit is formed between a via contact 102 and a MTJ unit 104. During the formation of the via contact 102, an etching process is performed to create a via on the top electrode 106 in the inter-metal dielectric layer 108, and the via is then filled with conductive materials to provide the via contact 102. As integrated circuits continue to scale down, the MRAM cells are susceptible to misalignment of the via contact 102, which increases the difficulty in controlling the end point of the via etching process, and therefore increases the possibility of a short circuit between the contact and the MTJ unit.
FIG. 2 illustrates a cross-sectional view of another conventional MRAM cell structure 200 where an undesired short circuit is formed between a direct contact 202 and a MTJ unit 204. Again, due to the difficulty in controlling the end point of an etching process, the front end of the direct contact 202 can extend beyond the top electrode 206 and reach the MTJ unit 204.
Such short circuits can cause the MRAM cell structures 100 and 200 to fail. Thus, what is needed is a MRAM cell structure that can avoid those undesired short circuits between the contacts and the MTJ units.